It depends upon what you are testing and what degree of accuracy is need. One needs to be aware of the shortcomings of the hobby kits we use in order to better comprehend the results. In some cases I use the API kits mentioned. In other cases i have more expensive digital equipment I use.
Most hobbyists use their test kits to measure the nitrogen complex- ammonia, nitrite and nitrate. many do no have kits for other things besides pH. Things like GH and KH are often measured for us by a local store and we do not need to retest these things very often.
When it comes to plants things can become more complicated. As a rule the more high tech one's plant4ed set-up is, the more other things one might want to test- phosphate and co2 levels would be two. For low and moderate light plants these kits are not really needed.
There is one other aspect to testing about which most hobbyists are not aware. This has to do with what things might cause false or inaccurate results from our kits. A great example of this is the nitrate kit. Due to the reagent employed it is very important with many kits the shake the heck out of some of the bottles before testing. The reagent tend to come out of solution in the bottle and sink to the bottom. In some cases shaking alone is not enough, you need to bang the bottom of the bottle on a table before or even during shaking to insure the reagent is thoroughly mixed. it is also important to know that nitrate kits are least accurate between 0 and 20 ppm. Finally, the way the nitrate kit works is that it doesn't actually measure nitrate. What it does is to cause a chemical reaction that converts the nitrate to nitrite and then it measures that. As a result if one tries to test during cycling, one must also measure nitrite and then deduct that number from the nitrate number to get the actual nitrate level.
One note here about plants and testing. There is a method of fertilizing plants called the Estimative Index, EI for short. Basically this method says plants should never be short of anything they need. Therefore it is nest to provide enough of these to insure they do not run short. This is further controlled by large, regular water changes which permit one to reset the tank weekly or even twice a week in some cases. This method doesn't rely on testing as nothing should ever be short and no serious excesses are allowed to build up.
Sorry to be so long winded, but to make the most of testing one should understand it is not necessarily simple in some cases.
I would leave you with this thought, One can buy the API ammonia test kit for about $6 dollars or you could buy an advanced laboratory grade ammonia digital test device that runs $4,000 or more. There are many options between these two. What one needs to measure and how accurate the results need to be determines what sort of test device you use. I have run 20 tanks (both with and without plants) now for a number of years. I almost never test 19 of them for anything. The 20th tank has a $250 digital monitor which reads TDS (Total Dissolved Solids), pH and temp continually.
For the most part to have a thriving planted tank what you need to know about your water is the pH, plants cannot thrive in all pHs. thing about 6.5 to 8.5 as being a good range, Then plants need many things that show up in hardness measurements.
Have a look around this site, they are probably the worlds premier developer, grower and seller of aquarium plants and have been for decades. Their guides are superb. Bear in mind they are trying to sell their own products. However, their explanations are universal for sure. I have used their micro-nutrient fert. for 15 years and about 2 months ago to save time (and not caring about cost) I have begun using their comprehensive mix.